Neptune was discovered in 1846 thanks to mathematical calculations, before anyone saw it through a telescope.
Why it is true
Uranus was not following its predicted orbit exactly, as if something were pulling on it. The Frenchman Urbain Le Verrier calculated where this unseen planet should be and sent the position to Berlin. The same night he received the letter, Johann Galle found it less than one degree from the predicted spot.
Source: NASA Science
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